Circuit Switching
by
Dr. Nguyen Minh Hoang
hoang.dhbk@gmail.com
March 2007
References
Reference books:
Data Communications and Networking, 4/e, by Behrouz A. Forouzan, DeAnza College
(http://highered.mcgraw-hill.com/sites/0072967757/information_center_view0/)
Data & Computer Communications by William Stallings (http://williamstallings.com/DCC/DCC7e.html)


Introduction
How can we connect multiple devices?
Point-to-point vs.
Multipoint
Better solution  switching
Switches: hardware and/or software devices allowing temporary connections between two or more devices
Three methods: circuit, packet, and message
Example of a Switched Network
Circuit Switching
Creates a direct physical connection between two devices
Originally developed for voice traffic
Does not use point-to-point connections between every device to every other device
Instead, devices link to a switch which may in turn link to other switches
Allows for a dedicated connection for duration of transmission
Circuit Switching
Space-division switching
Time-division switching
TDM Bus
Combinations
Circuit switching: create a direct physical connection between 2 devices such as phones or computers
Circuit Switch vs. Folded Switch
n inputs, m outputs
n lines in full duplex
Folded Switch: An n-by-n folded switch allows every device to connect to every other device in full-duplex mode
Space-Division Switch
The paths in the circuit are separated from each other spatially
Originally for analog networks, but now for both analog and digital
Crossbar switch
Multistage switch
Crossbar Switch
Inputs and outputs connected in a grid using electronic microswitches (transistors) at crosspoints
Disadvantage???
Too many crosspoints, only a few used at any given time
Inefficient; fewer than 25% of switches are in used at a given time
Multistage Switches
Combine crossbar switches in several stages
Devices are linked to switches that are linked to a hierarchy of other switches
Middle stages usually have fewer switches than first and last stages
Fewer crosspoints but still allowing multiple paths through the network  increased reliability

Multistage Switch (1)
A set of crossbar switches in several stages
Use a 15x15 crossbar => 225 crosspoints
Use the above multistage =>78 crosspoints
Multistage Switch
Multiple paths available for each pair of devices => reliable
Blocking problem when heavy traffic, experienced in public phone systems
Disadvantage???
Blocking
Reduced number of crosspoints may mean that at times of heavy traffic, an input may not be able to connect to an output if there is no path available (i.e. all switches are occupied)
Does not occur in single-stage switch; a non-blocking path is always available
Time-Division Switches
Uses time-division multiplexing to achieve switching
Used by most modern switches
TD switching involves partitioning low speed bit stream into pieces that share higher speed stream
Two methods:
Time-slot interchange (TSI)
TDM bus
Based on synchronous time division multiplexing (TDM)
Each station connects through controlled gates to high speed bus
Time slot allows small amount of data onto bus
Another line’s gate is enabled for output at the same time

TDM with a Time Slot Interchange
Without a TSI, order is unchanged
1->3, 2->4, 3->1, 4->2
Time Slot Interchange (TSI)
#memory locations = # inputs
size of mem. loc. = size of a time slot
Cannot handle bursty traffic
Consists of RAM with several memory locations
Fills up with incoming data from time slots in order received
Slots are sent out in order based on control unit logic
TDM Bus
Input and output lines are connected to a high-speed bus through input and output gates
Control unit opens and closes gates as needed
Time vs. Space Division Switch
Time division
No crosspoints
Delay
Space division
Many crosspoints
Zero delay

Space- and Time-Division Switching Combinations
Advantage of space-division switching is it is instantaneous; however, requires too many crosspoints
Advantage of time-division switching is it needs no crosspoints; however, processing connections creates delays
Combining them optimizes the number of crosspoints and reduces the amount of delay
TSS: Time-Space-Space
TSST: Time-Space-Space-Time
STTS: Space-Time-Time-Space
TST Switch
Telephone Networks
1876: Alexander Graham Bell patented the telephone
1878: Bell Telephone Company first operated
1984: AT&T breakup => AT&T Long Lines and 23 Bell Operating Companies, and a few other pieces
Since 1996: Any company can provide local or long-distance services
Telephone Network Structure
~22,000 end offices in US, each having up to 10,000 local loops
Local loop: analog 1-10km => how far is it if we stretch out all the local loops in the world?

Trunks between switching offices: coax, microwave, fiber optics
All trunks and switches are now digital
Major Components
Local loops – twisted-pair cable connecting subscriber phone to nearest end office or local central office
Bandwidth of 4000 Hz (4 KHz)
Trunks – transmission media that handles communication between offices
Muxing through optical fibers or satellite links
Switching Office – connects local loops or trunks allowing connections between subscribers
Major Components
LATAs
Local Access and Transport Area – defines the boundary of a service area
Intra-LATA services are provided by local exchange carriers (LEC)
Incumbent local exchange carriers
Competitive local exchange carriers
Communication is handled by end switches and tandem switches
Services
Inter-LATA services are handled by interexchange carriers (IXCs) – long-distance companies
To provide these services, each IXC must have a POP (Point of Presence) inside each LATA
Local Access Transport Areas (LATAs)
200+ LATAs in the US
Intra-LATA services
Services offered by the common carriers (telephone companies) inside a LATA
Such a carrier = local exchange carrier (LEC)
Before 1996: 1 LEC in a LATA => monopoly
After 1996: 1+ LEC inside a LATA
The LEC that owns the local loops system is called incumbent LEC (ILEC) => provide main services
Extra carrier = competitive LEC (CLEC) => extra services: mobile, toll calls, etc.
Switching Offices in a LATA
Calls that go through a tandem office is charged
Calls that are completed by using only end offices are toll-free
Inter-LATA Services
IXCs: inter-exchange carriers, a. k. a. long-distance companies, provide communication between 2 customers in different LATAs
AT&T, MCI, WorldCom , Sprint, Verizon
IXCs provide general data communications services including telephone. Phone service is normally digitized

How do IXCs interact with LECs? Use POP
Point of Presence (POP)
Example PSTN Network
Representative Voice Network Hierarchy
Example PSTN Network
Making Connections
Subscriber telephones are connected through local loops to end offices
Accessing the switching station at the end offices is accomplished via dialing
Dialing
Before: use Rotary or pulse dialing
for each number dialed, a digital signal is sent to end office
Prone to errors due to inconsistency of humans during dialing
Today: touchtone dialing
Rotary and Touchtone Dialing
Touchtone: Number 8 = 2 bursts of signals with f1=1336Hz and f2=852Hz
Analog Switched Telephone Services
Familiar dial-up service, analog local loop
Local calls
Toll call
intra- (when LATA is large) or inter-LATA
800 (888, 877, 866) services:
organization pays for call
Wide area telephone service (WATS):
Organization pays for outbound call at rate cheaper than long distance
900 services
Caller pays for call, much more expensive than long-distance
Analog Leased Lines
A line is dedicated permanently between 2 customers
This line goes through a switch, but seen as a single line because the switch for it is always closed.
No dialing is needed

Digital Telephone Services
Less sensitive to noise and other interference
Switch/56
digital version of analog switched line, data up to 56Kbps
Line is digital, no need for modem, but need a digital service unit (DSU)
Support bandwidth on demand, can use more lines for more bandwidth => can use video conferencing, fast fax, fast data transfer, among other services
Digital Data Service (DDS)
Digital version of analog leased line, up to 64Kbps
High-speed digital: T-lines and home networks
Question?